The present invention relates to devices for damping the vibrations of a cable used in the structure of a construction work, in particular a stay.
The invention applies in particular to stayed bridges. The stays are then anchored at their ends, for example to a tower and to the deck of the bridge. They thus support and stabilize the structure.
In some conditions, particularly when they are subject to periodic excitations, the stays may accumulate energy and oscillate considerably. The two primary causes of these vibrations are the movement of the anchorages under the effect of traffic loads or of the wind, and the effect of the wind acting directly on the cables. These oscillations may make the users anxious. In addition, if they are not controlled, they risk damaging the stays.
Several types of dampers are known. There are external dampers and internal dampers.
External dampers usually use piston-type dampers, of dimensions similar to those used for lorries or trains. These dampers are able to absorb energy when there is movement of their ends. One of these ends is attached to the cable, either directly via a collar, or via a pendulum in turn articulated on a collar attached to the cable. The other end of the damper is attached to a frame rigidly connected to the structure, usually the deck of the stayed bridge.
Internal dampers, for their part, are placed around the stay cable. They are usually situated in the extension of the tubes surrounding the bundle of metal strands making up the cable and attached rigidly to the structure (anchoring tubes for example). They act on the relative movements between the bundle of strands of the cable and the anchoring tube surrounding the bundle of strands when the cable vibrates.
Several damping principles are employed by internal dampers to dissipate energy:
/a/ by pouring a highly viscous oil into an annular trough situated around the bundle of metal strands of the cable and in which trough is mounted a ring that is transversely movable (see EP 0 343 054);
/b/ by distortion of a dissipating material, such as rubber, situated around the bundle of metal strands of the cable (see EP 0 914 521);
/c/ by dry friction between metal elements (see EP 1 035 350).
These internal dampers have the advantage of being discreet, hence more aesthetic than external dampers. The absence of anything bearing on the structure outside the anchoring tubes also simplifies the design of the work.
Nevertheless, the effectiveness of internal dampers is limited. Specifically, in the dampers operating according to principle /a/, the presence of viscous oil requires the use of sealed reservoirs of the bladder type which have limited resistance to high pressures. The dampers operating according to principle /b/ have low damping capability, limited by the performance of the materials available. Finally, in the dampers operating according to principle /c/, wearing of the contacting metal elements is inevitable and leads to loss of clamping and hence a reduction in the effectiveness of these dampers. The latter must therefore be periodically overhauled and adjusted.
One object of the present invention is to restrict the drawbacks of the existing dampers as listed above.